1. Field of the Invention
The present invention relates to a wavelength conversion device and a wavelength conversion method, more specifically to all-optical wavelength conversion used in optical communication systems.
2. Description of Related Art
A technique of performing signal processing directly on an optical signal without converting the optical signal into an electric signal is a common technique with all-optical signal processing, and further is important for optical communication systems.
Hereinbelow, a differential phase shift keying (DPSK) signal regenerator in the related art is described with reference to FIG. 8. For example, a general SPSK regenerator is disclosed in Masayuki Matsumoto, “3R regeneration of a DPSK signal utilizing a non-linear effect of a fiber,” General Conference of The Institute of Electronics, Information and Communication Engineers, 2006, B-10-22 (hereinafter abbreviated as “Matsumoto”).
A DPSK signal that is input into a DPSK signal regenerator 100 is divided into two signals. One of the divided signals is transmitted to a delay interferometer 105 and the other divided signal is transmitted to a clock regenerator 180.
The delay interferometer 105 converts the DPSK signal into an on/off keying (OOK) signal. The OOK signal generated by the delay interferometer 105 is transmitted to an all-optical wavelength converter 110.
In the all-optical wavelength converter 110, a wavelength of the OOK signal is converted and an amplitude of an optical signal is stabilized. The wavelength-converted OOK signal, which has a wavelength converted by the all-optical wavelength converter 110, is transmitted to a phase modulator 190.
Meanwhile, the clock regenerator 180 extracts a clock signal from the DPSK signal to generate an optical clock pulse signal, and then transmits this optical clock pulse signal to the phase modulator 190.
The phase modulator 190 includes a dispersion flattened fiber (DFF), which is a highly non-linear fiber. The wavelength-converted OOK signal and the optical clock pulse signal are both input into the dispersion flattened fiber (DFF). These two signals are cross-phase modulated (XPM) while propagating through the dispersion flattened fiber, and, as a result, a phase modulation pattern coinciding with an intensity modulation pattern in the wavelength-converted OOK signal is superimposed on the optical clock pulse signal. As a consequence, a wavelength-converted DPSK signal is output from the phase modulator 190.
The all-optical wavelength converter 110 includes an optical amplifier 142, a dispersion flattened fiber (DFF) 146 functioning as a highly non-linear fiber, and an optical band-pass filter 148. The configuration and operation of this all-optical wavelength converter 110 is described with reference to FIG. 9 to FIG. 11D below.
FIG. 9 is a schematic diagram showing a configuration of the all-optical wavelength converter 110. FIGS. 10A to 10C and FIGS. 11A to 11D are diagrams illustrating the wavelength conversion using the all-optical wavelength converter 110.
The optical amplifier 142 amplifies the input OOK signal (indicated by arrow S141 in FIG. 9) to generate an amplified signal (indicated by arrow S143 in FIG. 9 and FIG. 10A). The dispersion flattened fiber 146 expands a wavelength spectral width of the amplified signal S143 to generate a DFF output signal (indicated by arrow S147 in FIG. 9). Optical band-pass filter 148 has a wavelength pass-band having a different central wavelength from that of input OOK signal S141 (FIG. 10B). Accordingly, a converted OOK signal (indicated by arrow S149 in FIG. 9) is output from the optical band-pass filter 148 having a central wavelength different from that of the input OOK signal S141 by a wavelength shifted amount of Δλ (FIG. 10C).
Hereinbelow, a relationship between the signal intensity of the amplified signal S143 and the wavelength spectral width of the DFF output signal S147 is described with reference to FIGS. 11A and 11B.
The DFF output signal indicated by II in FIG. 11B is obtained by self-phase modulation induced by the amplified signal (indicated by S149 in FIG. 11A) in the dispersion flattened fiber 146. Here, the wavelength spectral width of the DFF signal becomes wider (as indicated by I in FIG. 11B) when the signal intensity of the amplified signal is increased (as indicated by I in FIG. 11A). By contrast, the wavelength spectral width of the DFF output signal becomes narrower (as indicated by III in FIG. 11B) when the signal intensity of the amplified signal is decreased (as indicated by III in FIG. 11A).
In addition, as shown in FIG. 11B, the dispersion flattened fiber 146 allows a flat wavelength spectrum to be obtained. Accordingly, the dispersion flattened fiber 146 makes the intensity of the DFF signal substantially constant even if the input signal has intensity fluctuations, and thus can remove any noise components while suppressing an effect of the intensity fluctuation of the input signal.
FIG. 11C shows a time domain waveform of the amplified signal. Meanwhile, FIG. 11D shows a time domain waveform of the wavelength-converted OOK signal S149 to be output from the wavelength converter. A noise component indicated by IV in FIG. 11C is not included in the time domain waveform of the wavelength-converted OOK signal S149 (FIG. 11D).
With this characteristic, the all-optical wavelength converter 110 functions not only as a wavelength converter but also as a discrimination circuit.
However, when a wavelength converter has to convert wavelengths of a high-speed optical signal having a data rate of 40 Gbps or more using a dispersion flattened fiber, its performance in the waveform reshaping function on the signal is significantly reduced because of the volume of the wavelength conversion.
To address this problem, the DPSK signal regenerator disclosed in Matsumoto includes multiple wavelength converters connected to one another. In each wavelength converter, an amount of wavelength conversion is adjusted such that the waveform reshaping function of the wavelength converter cannot be reduced. To be more specific, in order to achieve wavelength conversion of a shift amount of 10 nm, the DPSK signal regenerator includes five wavelength converters connected to one another with each having a wavelength shift amount set to 2 nm.
This makes it difficult to downsize the DPSK signal regenerator and also makes the DPSK signal regenerator not economically advantageous.
Furthermore, as the data rate increases, the width of the optical pulse needs to be narrower in proportion to a transmission rate, and thus, a dispersion value for the dispersion flattened fiber needs to be set smaller. An appropriate dispersion value required for the dispersion flattened fiber is proportional to a square of the pulse width, in other words, inversely proportional to a square of the data rate. Accordingly, if the data rate is increased four times, that is from 40 Gbps to 160 Gbps, the dispersion values required for the dispersion flattened fiber is reduced to 1/16. This means that the required dispersion value is −0.03 ps/nm/km, for example, which is extremely smaller in absolute value than a dispersion value −0.5 ps/nm/km of the fiber used in Matsumoto. It is difficult to manufacture a dispersion flattened fiber with such a dispersion value in its absolute value.